Downlink Cell-Free MIMO using Distributed Integer-forcing

We study a downlink cell-free MIMO (cf-MIMO) system consisting of K users, a central unit (CU), and N distributed base stations (BSs). The CU is connected to N BSs via N independent fronthaul (FH) links, which collectively serve K users. We put forth a distributed integer-forcing approach. The CU selects some integer coefficients and carries out a message-level precoding (MLP) over the K users' message sequences. The resultant MLP streams are forwarded to the BSs via the FH links. Then, the BSs perform signal-level precoding for the streams using distributed integer forcing (d-IF). The resultant signals of the N BSs are broadcast to the K users simultaneously. We derive the achievable rate of d-IF. Numerical results show that and the gap between the proposed d-IF and the capacity upper bound of the cf-MIMO is almost closed, while d-IF exhibits an improvement of more than 5 dB over existing distributed zero-forcing.